Negative feedback amplifier



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H. ROMANDER 2,480,163 y NEGATIVE FEEDBACK AMPLIFIER Filed April 14, 19453 Sheets-Sheet l I] V14 5 our/Our s 2 W M14/N AMPM/75? l/ 4 I j@ 6 y 3IZ IN VEN TOR. H060 /Po/w/f/voE/P BY am v1.30, W49. H. ROMANDER n2,430,163

NEGAT IVE FEEDBACK AMPLIFIER Filed April 14, 1945 s sheets-sheet 2 INVEN TOR. HUGO ROM/INDE? Mgg 309 m9.

Filed April 14, 1945 H. ROMANDER NEGATIVE FEEDBACK -AMPLIFIER 3Sheets-Sheet 3 IN V EN TOR.

HUGO ROMA/mf# AGENT Patented Aug. 30, 1949 NEGATIVE FEEDBACK AMPLIFIERHugo Romancier, North Caldwell, N. J., assignor to Federal Telephone andRadio Corporation, New York, N. Y., a corporation of DelawareApplication April 14, 1945, Serial No. 588,269

6 Claims.

This invention relates to wave amplifying systems and in particular toamplifiers employing negative feedback. Specifically the inventionrelates to a method of preventing self-oscillation in amplifiers of thistype.

It is well known that in feedback amplifiers the phase shift which wavesof different frequencies undergo during one complete excursion of theirtravel around the so-called a path varies for the different frequencies.It is usually desirable to so design an amplifier that al1 wavestraversing the a path over a given range of frequencies undergo a phaseshift of substantially 180 or as close thereto as possible. However,even though these conditions are fulfilled, there are always somefrequencies outside of the given range for which the phase shift becomeszero, 360 or a multiple thereof, and if the amplifier gain at thesefrequencies is greater than unity the amplier may sing orself-oscillate.

It has been suggested that amplifier self-os-cillation may be preventedby including within the amplier circuits, and usually in the feedbackpath, a network so designed that the waves which tend to produceself-oscillation are highly attenuated thus reducing the gain at thesefrequencies. However, with my invention self-oscillation at allfrequencies is prevented in accordance with a new principle whereinthere is produced in addition to the -conventional feedback voltage, anadditional voltage having such magnitude and phase that the resultantfeedback voltage is always displaced 180" with respect to the inputvoltage.v It is immaterial whether the waves which traverse theamplifier are originally impressed thereon from an outside source or arethe result of modulation or distortion components developed within theamplifier.

One of the primary purposes of employing negative feedback is to reducedistortion components developed within the amplifier. With the circuitarrangements of my invention the distortion components are mosteffectively reduced in the frequency range where the deviation from thedesired 180 phase shift for one complete excursion around the MS path isrelatively small, that is, of the order of 30 or less. However, since itis at these frequencies that distortion due to hum voltages occur, mycircuit arrangement is very effective in reducing this type ofdistortion. Negative feedback may be applied almost without limit withthe result that the h-um voltages are practically eliminated and stillthe amplifier will not oscillate at any frequency.

A primary object of my invention is to provide a stable negativefeedback amplifier circuit arrangement which permits high gain overanextremely wide frequency range.

Other -objects are:

To provide an improved negative feedback amplier in which the phasedifference between an applied input voltage and the net input voltage issubstantially zero.

To provide an improved negative feedback amplifier in which distortioncomponents are suppressed over a limited frequency range while theamplifier remains non-oscillatory over an extremely wide frequencyrange.

To provide a negative feedback amplifier having an overall gain which isproportional only to the forward amplification or It.

To provide a negative feedback amplier in which the ratio of the net oreffective input voltage with feedback to the applied voltage isconstant.

Other objects and features of my invention will become apparent from thefollowing description, the appended claims and the drawings in whichFig. 1 is a schematic diagram illustrating a preferred embodiment of myinvention;

Figs. 2, 3, 4 and 6 are vector diagrams which illustrate the principlesof operation of my invention;

Fig. 5 is a polar diagram showin-g the ,variation of m8 with frequency;and

Fig. '7 is a schematic diagram illustrating a modification of Fig. l;

Referring to Fig. 1 the reference character I represents a source ofvoltage which is to be amplii-led. This source may be of a singlefrequency or of a plurality of frequencies such as would occur in theoutput of a voice frequency amplifier. Although the source isillustrated as an alternator it could also be the secondary winding ofan input transformer or the output circuit of a vacuum tube amplifier.The reference character 2 represents the impedance of the source I andmay be, for example, a separate resistance as shown or the plateimpedance of an amplifier. The resistance 2 is connected to a junctionpoint 3 and to the grid 4 of an amplifying tube 5. The amplifying tube 5is illustrated as a triode for simplicity although tetrodes or pentodescould be employed without departing from the principles of my invention.The cathode 6 of the tube 5 is connected to ground through a biasingresistor 1. The anode 8 is connected to a main amplifier 9 illustratedin block form. The internal construction of the amplifier is immaterialin so far as my invention is concerned, It may b e a` single or amultistage amplifier; if the latter, the various stages may betransformer coupled, resistance-capacity coupled or by any other formsof coupling. Local negative feedback circuits may exist within theamplifier 9 and the phase shift of the amplifier at various frequenciesmay have any values whatsoever a1- though it would be preferable toemploy the best practice and make the phase shift other than the 180required for negative feedback as small as economically desirable. Theamplifying tube is to be considered as part of the amplifier 9 but it isshown separately for purposes of description.

It is understood, of course, that for an unbalanced negative feedbackamplifier such asis illustrated in Fig. 1 and where therev issubstantially zero phase shift in the feedback path, a phase reversal ofthe voltages for the frequencies within the operating range must occurin order to reduce distortion in accordance with the principles ofnegative feedback. All of the phase reversal need not take place intheforward or n path; some of it may be in the feedback or path.

In amplifiers of the push-pull type the phase reversal may be obtainedby crisscrossing the feedback connections from the output to the inputcircuits in accordance with known practice. The principles of myVinvention apply equally well to amplifiers of this type.

The necessary power supplies for operation of the amplifier are notshown` but are understood to be within the amplifier 9.

The output impedance of the amplifier 9 is illustrated as a resistor I0.A lead II connects the output of the amplifier to a load circuit, notshown. From a variable tapv I2 on the resistor Ill a negative feedbackconnection comprising a blocking capacitor I3 and a resistor I4 connectswith the junction point 3. As described so far the circuit arrangementconstitutes a typical form of feedback amplifier such as may be found inthe prior art. The gain of the amplifier at various frequencies maybesuch that the amplifier will oscillate at some frequency, perhaps notin the range for which the amplifier is designed but at some higher orsupersonic frequency at which the phase rotation through the amplifierand feedback path is 0, 360' or a multiple thereof as above explained.

In accordance with my invention I provide a circuit arrangement, alsoconnected to the junction point 3', which will produce a voltage havingthe proper magnitude and phase which, when combined with the feedbackvoltage of the main amplifier 9, will always produce a resultantfeedback voltage having a 180 phase displacement with respect to theinputvoltage. The circuit arrangement is so designed that it willfunction in the desired manner at any frequency whether this frequencybe that of the input voltage or of a distortion voltage developed withinthe amplifier 9. This circuit arrangementA will now be described.

From the junction point 3 a conductor I5 leads to the input grid I6 ofamplifying tube I1. The cathode I8 of this tube is connected to groundthrough a resistor I9'. TheV anode: 20 connects to what I have termed acorrelativo amplifier 2|, also shown in. block form. The tube I1 is tobe considered as part of thecorrel'ative amplier but is separately shownforv purposes of description. The output of the correlative amplifier isconnected to an output impedance shown as a resistor 22. A power supply,notl shown, is to be considered as connected to the tubes Within the 4correlativo amplifier. If desired the power supply for both amplifiers 9and 2I may be common.

The characteristic feature of the correlativo amplifier 2i is that itshould have substantially identical gain and phase shift characteristicswith respect to frequency as does amplifier 9.

The feedback connection comprising capacitor I3 and resistor I4 merelyimpresses a predetermined fraction of the output voltage developedacross resistor I@ onto the input of the main amplifier. This fractionis a scalar quantity, that is, no phase shift is involved. For thisreason a similar feedback circuit is not shown in connection with thecorrelativo amplifier 2|, any difference in voltage developed on theoutput resistor 22 due to this omission being compensated for by asuitable adjustment of the contact 23 which selects the required outputvoltage. On the other hand, if the feedback connection associated withthe main amplifier is such that a phase shift occurs in this connection,a corresponding circuit producing the same phase shift should beassociated with the correlative amplifier.

There may be cases wherein the construction of the correlatlve ampliermay be simplified by, for example, the omission of a stage ofamplification which involves nothing more than a phase shift of Thisphase shift could be compensated for by reversing the connections of atransformer within the amplifier or by other suitable means.

In practice amplifier 9 would usually comprise tubes capable ofproducing considerably higher output than the tubes in amplifier 2|. Thevariance in power output between amplifiers 9 and 2i is permissiblesince amplifier ZI merely supplies a voltage of desired magnitude andphase, and the value of this voltage, as determined by the point ofcontact 23 of conductor 24 on resistor 22, may be the total outputvoltage of the amplifier, whereas in conventional feedback practice thefeedback connection at contact I2 on resistor I0 in the output ofamplifier 9 is usually a very small percentage of the output Voltage.The essential point is that the gain and phase shift characteristicswith respect to frequency, from the junction point 3 to the output ofamplifiers 9 and 2I respectively, be substantially identical.

There is one essential difference between ampliers 9 and 2l and thispertains to their manner of operation rather than to their structure.The amplifier 9, being in fact a power amplifier is usually operated athigh efciency with the result that the amplifying tubes of saidamplifier operate over a large portion of their characteristics, and iftheir characteristics are not linear, distortion and modulation productsare produced. On the other hand it is not necessary that amplifier 2loperate at high efficiency. The tubes of this amplifier should operateonly over the linear portion of their characteristics, it beingessential that no appreciable amplitude distortion occurs within theamplifier.

Conductor I5 also connects junction point 3 with the grid 25 of tube 29,this latter tube consistuting together with tube 2l a phase mixer 28.The grid 29 of tube 2l is connected with contact 23 of resistor 22 bythe conductor 2li. The cathodes 30 and 3l of mixer tubes 26 and 21 areconnected to ground through resistors 32 and 33 respectively, theseresistors being shunted by capacitors 34 and 35. The resistors 32 and 33develop the desired negative biases for the grids of their respectivetubes. Resistors 36 and 31, connected to the anodes 38 and 39 of mixertubes 26 and 21 respectively, are for isolating the output voltages ofthese tubes so that they will not interact one with the other but willcombine to form a resultant voltage having a value which is the vectorsum of the voltages developed by tubes 26 and 21. This resultant voltagewhich appears at junction point 4U is amplified by the two-stageamplifier 4i. Resistance coupling is employed in this amplifier in orderthat a rninimum of phase rotation of voltages will occur therethrough.Two stages are employed in order that the phase of the voltagesdeveloped in the output of amplifier 4| will be the same as thoseapplied to the input; The output voltage of amplifier 4I is developedacross the resistor 42 in the plate circuit of the second stage. From avariable contact 43 a conductor 44 connects to the junction point 3through' blocking capacitor 45 and resistor 46. The impedance ofconductor I5 is assumed to be negligible.

In the above discussion -reference has been made to resistors 2, I4 and46. These resistors permit the use of a single grid on which thelcombined voltages of three separate sources may be impressed. It isassumed that the resistances are all equal and have values sufficientlygreat to permit the generation of voltages at each source uninfluencedby the voltages generated at the other two sources. In order to simplifythe explanation of my invention I have assumed that the resistances 2,i4 and 46 are all equal so that the three voltage sources each haveequal opportunities to iniiuence the net voltage on grid 4, although anincrease in any of the resistors can be compensated for by aproportionate increase in the respective source voltage. Moreover, suchphase rotation as may exist in practice due to tube and straycapacitance at point 3 to ground is disregarded since this effect hasbeen found negligible over the frequency range in which self-oscillationdue to feedback around the main amplifier is possible.

The operation of the circuit of my invention will now be explained bymaking reference to the vector diagram shown in Fig. 3. In the followingdiscussion of feedback amplifiers, and in particular such amplifiersutilizing my invention, certain mathematical notations and expressionswill be used and are here defined. Reference characters found in thedefinitions refer to Fig. 1.

Notation or Expression Denition The voltage gain of the main amplifierwithout negative feedback. This is a vector quantity, having bothamplitude and phase angle.

The factor by which the voltage output of an amplifier employingfeedback must be multiplied to obtain the voltage which is impressedback onto the input of said amplifier. This, also, is a vector quantity.

The phase angle of the product p3.

A reference frequency, generally Within the useful frequency range ofthe amplifier, at which qS is 180.

A rfaegice frequency at which 15 is zero or a multiple In Fig. 2 I haverepresented by vectors the system of voltages which add vectorially toproduce a net voltage V at the grid 4 of amplifying tube 5. Thus, at anygiven frequency, there will exist simultaneously the applied oractuating voltage V1, the feedback voltage V2=p/8V, the phase-mixervoltage V3 and the voltage V which is the vector sum of V1, V2 and V3.That is,

The significance of V3 in this equation may be illustrated by referenceto a conventional feedback system operating without benefit of V3. Thevector diagram of such a system is shown in Fig. 3, Where the amplitudeand phase angle of a is the same as for Fig. 2. It is a well-known factthat if qb is 0 (or 360) and the ratio of V2 to V is greater'than unity,the conventional feedback system will become unstable; that is, it mayself-oscillate. One purpose of my invention is to avoid this unstableoperation or self oscillation in a negative feedback amplifier.

Again referring to Fig. 2 in which the magnitude and phase angle of a isthe same as for Fig. 3, a stable condition of operation may be achievedby developing a voltage represented by vector Vs having a magnitude andphase angle for all frequencies such that, when added to voltage vectorV2, a resulting or net feedback voltage vector V4 will be produced whichwill always be substantially from V1 and have an amplitude which is axed fraction of thev amplitude of V1 for all frequencies. Such a systemcannot oscillate, since for any value of i the net feedback voltage willalways have substantially the amplitude and phase angie as thatcorresponding to operation at mid-frequency at which qs equals 180.

The manner in which the voltage represented .by vector Vs is developedwill now be explained by referring to the vector diagram of Fig. 4. Thisfigure represents the phase and amplitude relations between the voltagesimpressed upon the inputs of the phase mixer 28, the voltages developedtherein, and the resultant or output voltage of the phase mixer. The nal(net) voltage, represented by vector V and impressed on the grid 4 oftube 5, is also impressed upon the grid I6 of amplifying tube I1 andupon the grid 25 of tube 26, the latter constituting one element of thephase mixer 28. The voltage impressed upon grid i6 appears, afteramplification in the correlative amplifier, across resistor 22. Thisvoltage will have a phase relation identical with that appearing acrossresistor il] in the output of the main amplier 9, it being rememberedthat the correlative amplifier is so designed that its gain and phaseshift-frequency characteristic is substantially the same as that of themain amplifier. The contact 23 selects the desired magnitude of thisvoltage which is impressed on the grid 29 of tube 21, the latterconstituting the second element of the phase mixer 28. This voltage isrepresented by vector Ve on Fig. 4.

There are now two voltages, V and Vc, operating on the phase mixer. TheVoltage V is amplified and reversed in phase by the tube 26 and appearsin the output circuit thereof between the point 40 and ground. Thisamplified voltage is represented by vector V5 in Fig. 4. Voltage Vc isamplified and reversed in phase by the tube 21 and also appears in thetube output circuit between the point 40 and ground. This amplifiedvoltage is represented by vector Vc. The resultant of the voltagevectors V5 and Va is Va.

The phase angle of V3' is dependent on the mag-2 nitude of Vc which inturn is determined by the position of contact 23 on resistor 22. Themagnitude of V3 is also dependent on the position of contact 23. Thislatter dependency is of particular importance for the condition where qsequals 180 as will be seen when the preferred circuit adjustments aredescribed.

The voltage represented by V3 is amplified by the amplifier 4I andappears without change of phase across resistor 42. Contact 43determines the magnitude of the voltage which is impressed back on thegrid 4 of tube 5, and has already been defined as vector V3.

To facilitate the desired adjustment of contact 23 on resistor 22 it ispreferable to make said adjustment when the applied voltage from sourcelf has the frequency for which =180, that is, mid-frequency. Thisfrequency may be obtained by plotting a polar diagram of the magnitudeand phase angle of a over a range of frequencies sufiiciently great toinclude the mid-frequency. Since, when employing this frequency, thevoltage V3A should be zero, it is only necessary to place a suitablevoltmeter across the output of the phase mixer and make the properadjustment of contact 23 until this voltmeter reads zero. This followsfrom the fact that =180 and, in Fig. 4, Vc is in phase opposition to V.

The desired adjustment of contact 43 on resistor 42 may most readily bemade by applying a voltage from source l having a frequency for whichthe phase angle qs is zero, that is, a critical frequency. It ispreferable that the particular critical frequency selected be the nexthigher frequency above the mid-frequency. When employing this frequency,the voltage V3 should be of such magnitude as to reduce the output ofthe main amplifier to that value which corresponds to aVi (for thecritical frequency) divided by the numerical value of l-a atmid-frequency. This follows from the fact that when using thecorrelative amplifier and phase mixer in connection with a feedbackamplifier in accordance with my invention, the net gain of the feedbackamplier should be a (measured at a particular frequency) divided by thenumerical value of l-a measured at mid-frequency. A suitable voltmetermay be placed across resistor l and the position of contact 43 varieduntil the output voltage corresponds to that indicated above.

The value of this voltage can also be obtained by employing dataobtained from the above described polar diagram provided the daigrain isextended to include a frequency for which =0. This will be clear byreferring to Fig. 5 in which the curve represents the locus of a for arange of frequencies suiiicient to include a frequency for which q =180and a frequency for which =0.

When adjusting the contact 23 on resistor 22 to obtain zero output fromthe phase mixer as above described, a given voltage V1 at the frequencyfor which =180 is applied between junction point 3 and ground. Thisvoltage is not applied directly but is the result of a given voltagefrom source l and the known resistance values of resistors 2, I4, and46. This given voltage V1 may be assumed to be unity. When the output ofthe phase mixer is zero the output voltage of amplifier 9, as measuredacross resistor I0 or between the contact l2 and ground, is noted.

When adjusting the position of contact 43 on resistor 42 to produce therequired operating conditions, the frequency ofsource l is changed tothe value for whichl ==0. From the diagram of- Fig. 5 the ratio o f thescalar value of a for =O to the scalar value'of a for =180 is obtained.With the voltage V1 at the frequency for which Il' 1*#13 that is, theoutput. voltageis lf V The vector relations for this circuit are shownin Fig. 3. It is apparent that V will not remain constant either inposition or in magnitude for different frequencies, whereas inaccordance with my invention the vector V remains constant both inposition and magnitude for any value of a as shown by Fig. 2. Thesignificance of this fact is that the gain and phase-frequencycharacteristic of the conventional negative feedback amplifier isaltered by the feedback circuit, whereas, in accordance with myinvention, the gain and phase-frequency characteristic is unchanged bythe feedback circuit. In the latter instance, the gain is reduced by aconstant proportion corresponding to the degree of feedback establishedat mid-frequency and the output voltage is aV.

So far nothing has been said regarding distortion voltages which may bedeveloped in the main amplifier. It will now be shown that thesedistortion voltages will not produce self-oscillation and that withoutchange in adjustments for the correcting Voltage V3 the phase of thefinal distortion voltages will always coincide with the phase of thedistortion voltages as iirst applied to the grid 4 of tube 5.

'The distortion voltages first appear Within the circuits of the mainamplifier and are impressed upon the output resistor l0. A portion ofthese voltages, depending on the position of contact l2, are fed backonto the grid 4. The voltages thus impressed on grid 4 will set up asystem of voltage vectors in exactly the same manner as if thedistortion voltages originated in the source l The operation of thecorrelative amplifier and the phase mixer circuits produce' the requiredcorrecting voltage and a vector diagram of the various voltagesthroughout the circuit resulting from the distortion components will beidenticalwith that of Fig. 2, where V1 represents the contribution ofthe distortion voltage appearing in the amplifier output throughresistor i4 to the net voltage V.

In order to show the extent to which distor` tion is suppressed in thecircuit of my inven. tion reference is now made to the vector diagram ofFig. 6. In this diagram I have reproduced the vectors of Fig. 2 withoutchange in nomenclature except to substitute for V the referencecharacter D (for distortion).

In Fig. 6 the vector Di represents the distortion voltage at an assumedfrequency as it first appears on, or is applied'to, the input of thetube 5 at the junction point 3, it being remembered that distortionvoltages are rst generated within the ampiirler 9. In accordance withthe explanation already given of the operation of the correlativeamplifier and phase mixer, the result of this applied distortion voltagewill be a feedback voltage D2 and a phase mixer voltage D3 resulting ina net voltage D. It is here assumed, for simplicity, that B has noinherent phase angle, so that for the distortion frequency assumed thephase angle o1 is the phase angle of u.

Vectors D1 and D2, although representing voltages between the junctionpoint 3 and ground, are directly proportional to, and thereforerepresentative of, the distortion voltage first appearing across outputresistor Il] without feedback and to this voltage as modified byfeedback, respectively. The resultant distortion voltage is thereforeproportional to the vector sum of D1 and D2 or D. It will be seen,therefore, that in accordance with my invention the distortion has beenchanged in the ratio of vector D1 to vector D.

If the frequency of the distortion voltage developed Within theamplifier is such that the phase angle of fl=2 and the feedback voltageis represented by D2', the resultant distortion voltage is proportionalto vector D, that is, the sum of vectors D1 and D2. In this case it willbe evident that the final distortion with feedback is considerablygreater than the distortion without feedback. In other words, in so faras distortion components for frequencies having considerable phaseshifts are concerned, there may be no reduction in distortion due tofeedback.

It may be well to point out that the vectors D2 and D2 have been drawnsuch that they have the same magnitude as vector D4. Usually attenuationaccompanies increased phase shift with the result that in an actualamplifier the voltage vector D2 would be somewhat smaller than that ofD3, and the voltage represented by vector D2 would be smaller than thatof voltage vector Dz. If this were the case, the final distortionvoltage vector D" would not be as great as that shown in Fig. 4. In anycase, however, it is only the distortion components having small phaseshifts which are considerably reduced by the circuit arrangements of myinvention.

As shown in Fig. 6, if the feedback voltage maintains a constant Valueregardless of frequency, the locus of the ends of the vectorsrepresentative of the final distortion is a circle shown by the dottedcircle 41 having its center at the extremity of vector D1, and having aradius equal to D2.

One very objectionable distortion component encountered in practice isdue to the use of alternating current for heating the cathodes of thevacuum tube amplifiers and to the ripples in the rectified voltages forsupplying power to the tube anodes. These components usually occur atfrequcncies which are subject to only a small phase shift. It willtherefore be seen that the circuits of my invention are particularlyvaluable in reducing distortion components due to hum because anunusually great amount of feedback may be employed for reducing thesedistortion components without self-oscillation.

It may be desirable to more completely suppress a distortion componentat some particuular frequency. This may be done by inserting a phaseshifting network in the amplier circuits such that the phase angle of upwill be substantially 180 at the desired frequency.

Another advantage of the circuit arrangements of my invention is thatthey may be applied to any existing amplifier whose operation it isdesired to improve. All that is required is that a polar diagram showingthe variation of ,u with frequency be made of the existing amplifier,and a correlative amplier be constructed which has a ,a-frequencycharacteristic represented by substantially the same diagram. Thecorrelative amplifier 2 l, the phase mixer 28 and the amplifier 4|constitute an active network which may be connected to the circuits ofthe main amplifier at a single point, namely the junction point 3. Thereis no need of making any change to the amplifier circuits whoseIperformance is to beimproved other than, for example, the adding ofisolation resistors such as resistors 2 and I4, and these may already bepart of said amplifier.

Another manner in which the applied voltage, the feedback voltage, andthe phase mixer voltage may be caused to act independently is shown inFig. 7. In this ligure I have dispensed with the isolating resistors I4and 4B and have submitted a multigrid tube 48 for the triode 5 of Fig.l. Resistor 2 may now have la relatively low value without affecting theoperation of my invention. I have illustrated the tube 48 as havingthree control grids, 49, 5i] and 5I, one for each of the voltages whichis to independently exert control on the anode current of the amplifyingtube d8. A voltage representing the resultant of the combined effects ofthese voltages will be found to exist across the resistor 52 connectedbetween the cathode 53 and ground. The desired potential to be appliedto tubes l'l and 26 may be selected by contacts 54 and 55 respectivelyon resistor 52. It will be noticed that no phase change has been assumedbetween the-voltages applied to the grids of the tube 48 and thosevoltages obtained from the resistor 52. This will be substantially truefor the range of frequencies of greater interest. Resistors 56 and 51are grid leaks for establishing an appropriate D. C. potential on thegrids 5| and 49 of the tube 48.

Other modifications of my invention will occur to those skilled in theart and it is to be understood that the illustrations given in thisspecication are given by way of example only and not as limiting thescope of the invention as set forth in the objects and the appendedclaims.

In certain of the claims I have used the expression vector-frequencycharacteristic. A vectorfrequency characteristic is understood to be thevector quotient of the output voltage divided by the applied inputvoltage over a frequency range. The vector-frequency characteristictakes account of the phase angle relationship as well as the amplituderelationship between the output Voltage and applied input voltage.

What is claimed is:

1. In combination, a feedback amplifier having an input circuit and anoutput circuit and a feedback connection therebetween, a correlativeamplilier having an input circuit and an output circuit, and a. phasemixer having two input circuits and an output circuit, means connectingthe input circuits of said feedback land correlative amplifiers and oneof the input circuits of said phase mixer together at a common point,means connecting the output circuit of said correlative amplifier to theother input circuit of said phase mixer, means connecting the output ofsaid phase mixer to said first connecting means, a source of potentialapplied to said common point, and a load circuit connected to the outputcircuit of said feedback amplifier.

2. The combination of claim 1 in combination 11 -With means foradjusting the output voltage of said phase mixer to zero for a frequencyof said source of potentia1 for which phase shift through said feedbackamplifier is 180.

3. In combination, a feedback amplifier having an input and an outputcircuit and a feedback connection therebetween, a correlative amplifierhaving an input and an output circuit, a phase mixer having input andoutput circuits, means connecting the input circuits of the correlative.amplifier and phase mixer to the input circuit of the feedbackamplifier, means connecting the output circuit of the correlativeamplifier to the input circuit of the phase mixer, means connecting theoutput circuit of the phase mixer to the input circuit of the feedbackamplifier, a source of applied voltage having a range of frequenciesconnected to the input circuit of th'e feedback amplifier and voltageadjusting means for adjusting the magnitude and phase of the voltagedeveloped by the phase mixer to a Value such that the final voltageeffective in the input circuit of the feedback amplifier will have thesame phase as the applied voltage from said source over said range offrequencies.

4. The combination of claim 3 in which the in' put circuit of thefeedback amplifier comprises a multigrid tube, the applied voltage beingconfnected to a grid cf said tube, said feedback connection beingconnected toa grid of said tube; and the output of the phase mixer beingconnected to a grid of said tube.

5'.` A negative feedback amplifier comprising an input circuit and anoutput circuit and. al feedback connection therebetween for provid-ing a35 2,244,249

feedback voltage and an additional circuit connected across said inputcircuit for producing an additional voltage which when combined withsaid feedback voltage will produce negative feedback over a wide band offrequencies, said additional circuit comprising a phase mixer circuithaving two input circuits and an output circuit and a correlativeamplifier having an input and output circuit, said input circuit of saidcorrelative amplifier and one of said input circuits of said phase mixerbeing connected to the input of said amplifier, the output circuit ofsaid correlative amplifier being connected to the other input circuit ofsaid phase mixer, and said output circuit of said phase mixer beingconnected to said input circuit of said amplifier.

6; A negative feedback amplier according to claim 5, further comprisingadjustable means connecting said correlative amplifier to one of theinput circuits of said phase mixer for adjusting the magnitude and phaseof the voltage applied thereto such that the output Voltage of saidphase mixer is zero for frequencies for which the phase shift of saidfeedback voltage is HUGO ROMAN'DER.

REFERENCES' ciED The following references are of record in`` th file ofthis patent:

UNITED STATES PATENTS Number Name Date n, 1,994,486 Robertson Mar. 19,1935 2,227,048 West Dec. 31, 1940 Guanella June 3, 1941

